The mean moment of inertia of Mars is, at present, very poorly constrained. The generally accepted value of 0.365 MR2 is obtained by assuming that the observed second degree gravity field can be decomposed into a hydrostatic oblate spheroid and a nonhydrostatic prolate spheroid with an equatorial axis of symmetry. An alternative decomposition is advocated in the present analysis. If the nonhydrostatic component is a maximally triaxial ellipsoid (intermediate moment exactly midway between greatest and least) the hydrostatic component is consistent with a mean moment of 0.345 MR2. The plausibility of this decomposition is supported by statistical arguments and comparison with the Earth, Moon and Venus. If confirmed, this new value would have significant implications for the inferred composition and climatic history of Mars. The Mars Observer mission may help resolve this issue. ¿ American Geophysical Union 1989